Opaque Color Stack for Electronic Device

ABSTRACT

An opaque cover for a capacitive sensor is provided. The cover includes a transparent substrate and a black color stack disposed adjacent the transparent substrate. The black color stack includes a pigment stack having a first dielectric layer, a second dielectric layer, and a first light absorbing layer positioned between the first and second dielectric layers. The first dielectric layer has a first refractive index. The second dielectric layer has a second refractive index different from the first refractive index. The black color stack also includes a plurality of second light absorption layers interleaved with a plurality of third dielectric layer.

This application is a continuation of U.S. patent application Ser. No.14/019,526, filed Sep. 5, 2013, which is hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

Embodiments described herein generally relate to a coloring for an inklayer of an electronic device. More specifically, embodiments relate toa thin, opaque, non-conductive black coloring for an electronic devicethat may permit operation of a capacitive sensor therethrough

BACKGROUND

Many portable digital devices incorporate at least one display screen toprovide graphical information to a user or viewer. The display screenmay include a liquid crystal display (LCD). Such devices may alsoinclude one or more sensors located beneath a cover glass that overlies,and typically extends beyond, the LCD. As one example, these sensors maybe capacitive sensors.

The devices may also incorporate an opaque region, such as black coatingregion, outside the display screen (e.g., outside the active displayregion) but beneath the cover glass. The opaque region may includeopaque ink like black ink under a cover glass or sapphire. The devicesmay also incorporate a button, which is one non-limiting andnon-exclusive way to permit a user to provide input to the device. Whenthe button is implemented as or incorporating a mechanical switch, it isoften located within the opaque region. The same may be true when thebutton is a “soft” button, e.g., is a non-moving element that senses atouch and/or force exerted on a surface of the soft button.

A sensor, such as a capacitive fingerprint or touch sensor, may bepositioned under the button. Generally, the black ink should to be thinenough to make the sensor sensitive, but also optically opaque toconceal the sensor and match the coloring of the opaque region.

A black ink having these properties may include a high percentage ofcarbon pigments, such as carbon black, to obtain adequate opticaldensity. However, carbon pigments typically are conductive, which mayaffect the operation of a capacitive sensor located beneath the inklayer. Further, the relative thickness of the ink layer may increase thedistance between the sensor and an object it attempts to sense, such asa finger atop the button. Generally, the sensitivity of a capacitivesensor varies inversely with the square of the distance between thesensor and sensed object, so relatively small changes in distance mayhave large effects on sensor performance. Additionally, particles,voids, and contamination in the black ink or paints may affect theperformance of the sensor and cause functional errors in sensorreadings. These issues increase as the thickness of the ink layer usedto color the button increases. Therefore, a thinner, non-conductive (orless conductive) black ink may be useful.

SUMMARY

Embodiments described herein may provide a thin opaque non-conductiveblack color stack that makes a highly sensitive sensor, such as acapacitive sensor, underneath a cover substrate invisible. The sensormay provide a clean signal when the cover substrate, such as a coverglass or sapphire, is touched. The thin non-conductive black color stackis positioned between the cover substrate and the capacitive sensor. Thenon-conductive black color stack may include a non-conductivelight-absorbing stack, which includes a relatively thin layer of a lightabsorbing material, such as tin, and a relatively thick layer ofdielectric material, such as silicon nitride or silicon oxide. The tinlayer is kept under 100 nm thick in order to be non-conductive. Thenon-conductive black color stack may also include a top pigment stack,which includes two different dielectric layers sandwiched with a lightabsorbing layer, such as a tin layer.

The black color stack is non-conductive with a resistivity of at least10¹⁴ Ωcm and has a relatively low dielectric constant compared to thatof certain types of black ink, such as certain inks using carbon as apigment. The high resistivity and relatively low dielectric constant ofthe black color stack help improve the performance of the capacitivesensor. The black color stack is thin enough to allow the capacitivesensor to sense finger touching on the cover glass, while the blackcolor stack still has the optical density of at least 3 or greater to beoptically opaque to make the capacitive sensor underneath invisible.

In one embodiment, an opaque cover for a capacitive sensor is provided.The cover includes a transparent substrate and a black color stackdisposed adjacent the transparent substrate. The black color stackincludes a pigment stack having a first dielectric layer, a seconddielectric layer, and a first light absorbing layer positioned betweenthe first and second dielectric layers. The first dielectric layer has afirst refractive index. The second dielectric layer has a secondrefractive index different from the first refractive index. The blackcolor stack also includes a plurality of second light absorption layersinterleaved with a plurality of third dielectric layers.

In another embodiment, a method is provided for forming a black colorstack over a substrate. The method includes depositing a pigment stackover a transparent substrate. The pigment stack includes a firstdielectric layer separated from a second dielectric layer by a firstlight absorbing layer, the first dielectric layer and the seconddielectric layer having different refractive indexes. The method alsoincludes depositing a non-conductive light-absorbing stack over thepigment stack, and positioning a capacitive sensor adjacent to thelight-absorbing stack.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the embodiments discussed herein. A furtherunderstanding of the nature and advantages of certain embodiments may berealized by reference to the remaining portions of the specification andthe drawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of an electronic device inaccordance with embodiments of the present disclosure.

FIG. 2 illustrates a cross-sectional schematic view of an opaque regionof an electronic device in accordance with a first embodiment of thepresent disclosure.

FIG. 3 illustrates a cross-sectional schematic view of an opaque regionof an electronic device in accordance with a second embodiment of thepresent disclosure.

FIG. 4A illustrates a first cross-sectional schematic view of an opaqueregion of an electronic device in accordance with a third embodiment ofthe present disclosure.

FIG. 4B shows a sample embodiment of a black color stack.

FIG. 5A illustrates a cross-sectional schematic view of sample layers ofan opaque region of an electronic device, in accordance with variousembodiments of the present disclosure.

FIG. 5B illustrates a cross-sectional schematic view of sample layers ofan opaque region of an electronic device, in accordance with variousembodiments of the present disclosure.

FIG. 5C illustrates a cross-sectional schematic view of sample layers ofan opaque region of an electronic device, in accordance with variousembodiments of the present disclosure.

FIG. 5D illustrates a cross-sectional schematic view of sample layers ofan opaque region of an electronic device, in accordance with variousembodiments of the present disclosure.

FIG. 5E illustrates a cross-sectional schematic view of sample layers ofan opaque region of an electronic device, in accordance with variousembodiments of the present disclosure.

FIG. 5F illustrates a cross-sectional schematic view of sample layers ofan opaque region of an electronic device, in accordance with variousembodiments of the present disclosure.

FIG. 6 is a schematic diagram illustrating a capacitive sensor forsensing a fingerprint or finger touch, in accordance with embodiments ofthe present disclosure.

FIG. 7 is a flow chart illustrating steps for fabricating a displaycover in accordance with embodiments of the present disclosure.

FIG. 8 is a simplified diagram of a metal block in accordance withembodiments of the present disclosure.

FIG. 9 is a flow chart illustrating steps for applying a black ink layerto a substrate in accordance with embodiments of the present disclosure.

FIG. 10 shows application of a black ink layer to a substrate inaccordance with embodiments of the present disclosure.

FIG. 11 is a simplified system diagram for a sample deposition system.

DETAILED DESCRIPTION

The present disclosure may be understood by reference to the followingdetailed description, taken in conjunction with the drawings asdescribed below. It is noted that, for purposes of illustrative clarity,certain elements in various drawings may not be drawn to scale, may berepresented schematically or conceptually, or otherwise may notcorrespond exactly to certain physical configurations of embodiments.

The disclosure discusses a non-conductive black color stack that may bepositioned between a transparent cover or substrate, such as sapphire orglass substrate, and a sensor, such as a capacitive sensor, and throughwhich the sensor may operate to sense an element position on thesubstrate. The black color stack generally is positioned under thetransparent cover, and makes the capacitive sensor invisible. The blackcolor stack may include a pigment stack and an adjacent non-conductivelight-absorbing stack.

The pigment stack may include a first dielectric layer separated from asecond dielectric layer by a light-absorbing layer. The first dielectriclayer generally has a different refractive index from the seconddielectric layer. For example, SiO₂ has a different refractive indexfrom that of Nb₂O₅ or Si₃N₄. The pigment stack may adjust or tune itsblack color to any desired or useful shade of black. As one example, byvarying the materials and/or thicknesses for the two differentdielectric layers, the black color may be varied or tuned slightly, forexample, to produce a yellowish black, bluish black, or green blackcolor. In addition to using materials that have varying refractiveindexes, the thicknesses of the various layers may affect wavelengths oflight absorption.

The light-absorbing stack absorbs at least a portion of incident lightthat passes through the cover glass. The light-absorbing stack mayinclude a light absorbing material, such as tin, which has high lightabsorption. In some embodiments tin may be replaced by copper oxide(CuO) or zinc oxide (ZnO) or another light absorption layer which isless conductive than carbon fibers used in standard black pigments.

The non-conductive light-absorbing stack may also include dielectricmaterials. Specifically, the non-conductive light-absorbing stack mayinclude dielectric layers (e.g., Si₃N4, SiO₂, or Nb₂O₅) interleaved withlight absorbing layers (e.g., tin, CuO, or ZnO).

Each of SiO₂, Si₃N₄, and Nb₂O₅ has a relatively low dielectric constantcompared to the dielectric constant of certain type of black ink, suchas certain carbon-based inks having high dielectric constants. Sincetitanium oxide has a high dielectric constant, it acts to insulate asensed element touching or adjacent the substrate (such as the coverglass) from the capacitive sensor. Thus, alternative materials may beused to impart color in applications or embodiments where the coloredink overlies the sensor, or is otherwise positioned between the sensorand the sensed element.

The black color stack may be made as thin as 1 μm to 2 μm in certainembodiments, which may permit the sensor to capacitively couple throughthe ink layer to the sensed element.

FIG. 1 illustrates a perspective view of an electronic device inaccordance with embodiments of the present disclosure. Electronic device100 may include a display 102 on a surface, such as a top surface, todisplay information to users. In some embodiments, the display may betouch-sensitive.

The display may incorporate liquid crystal display (LCD) technology,which is one of the most widely used display technologies. An LCDgenerally includes a backlight module, such as a light emitting diode(LED), a bottom polarizer, a TFT glass, a liquid crystal layer, a colorfilter glass, and a front polarizer. The display may also utilizeorganic light emitting diodes (OLED), plasma technology, and the like inlieu of the LCD and/or backlight.

Electronic device 100 may be any of a variety of devices, most of whichemploy a hard substrate as a cover glass/window. For example, theelectronic device may be a mobile phone, tablet computer, laptopcomputer, instrument window, appliance screen and the like. It should beappreciated that the cover glass may be soft and somewhat flexibleinstead of hard and/or rigid, as in the case of certain laptopcomputers.

Electronic device 100 may include the aforementioned cover glass 112,which covers the display 102 and, typically, an opaque region 104surrounding or adjacent the display 102. In the opaque region, the coverglass 112 is partially printed with an opaque coating, such as a whitecoating or a black coating, or otherwise has such a coating appliedthereto. Cover glass 112 may be a transparent or may have a transparentportion to permit a user to view the display 102. For example, the coverglass 112 may be made fully or partially from sapphire, glass (which maybe chemically treated), certain plastics, certain polymers, and thelike.

The opaque region 104 typically is positioned outside the display 102 orthe active region of the display, although this is not necessary. Insome embodiments, a button or other input mechanism may be positioned inthe opaque region 104. The button 108 may act as an input mechanism tocontrol certain operations of the electronic device, or to provide inputto the electronic device. One or more capacitive sensors or sensingelements may be located underneath the button 108. An opaque ink layerand/or adhesive may be placed between the bottom surface of the button108 and the top surface of the sensor. The adhesive may bond the sensorto the button, while the ink layer may color the button. The ink layermay color the button to match the opaque region 104, for example. Boththe button and the opaque region 104 may be white, black or any othercolor.

One sample sensor may sense a finger touching or nearly touching thebutton 108 through a change in capacitance at the sensor or itsconstituent elements (if there are multiple sensing elements). When theelectronic device capacitively senses a touch on the button, forexample, the device may activate certain functions associated with thecapacitive sensor. As one example, the sensor may be a fingerprintsensor and may operate to capture a fingerprint when a touch is sensed.

Top cover glass 112 is supported by a housing 110. The housing 110 maybe formed of a variety of different materials including, but not limitedto, polymer materials (e.g. plastics), metals (e.g. aluminum, steel,titanium, alloys, and the like), amorphous glass materials, compositematerials, and combinations thereof.

FIG. 2 illustrates a cross-sectional schematic view of a button 108 inaccordance with one embodiment of the present disclosure. Thecross-section may be taken along line A-A in FIG. 1. Stack 200 includesa cover substrate 202, such as a cover glass or sapphire, on the top ofthe stack and a black color stack 204. A sensor 208 may be positioned atthe bottom of the stack. (In some embodiments, the sensor may beseparated from the stack 200 by an air gap.) The black color stack 204includes a pigment stack 206 having a first dielectric layer 204A, alight absorbing layer 204B, and a second dielectric layer 204C. Thepigment stack 206 can tune or adjust black color and help match thecolor under the button to the opaque region 104. The black color stack204 also includes a non-conductive light-absorbing stack 210 havingdielectric layers 204A interleaved with light absorbing layers 204B. Thelight absorbing layers absorb light to make the non-conductive lightabsorbing stack 210 darker, or more black in color. The light absorbinglayers 204A may include tin layers, which can be non-conductive when thetin layers are kept very thin. The dielectric layers 204A further helpensure that the light absorbing stack 210 is non-conductive. The blackcolor stack 204 has an optical density of approximately 3 or greater andabsorbs all incident light. The black color stack 204 is also relativelythin (e.g., less than 2 microns) and non-conductive.

FIG. 3 illustrates a cross-sectional schematic view of opaque region 104in accordance with a second embodiment of the present disclosure. Stack300 includes the cover substrate 202 on the top and a black color stack304 underneath the cover substrate. The black color stack 304 includespigment stack 306 having a top dielectric layer 304C, a light absorbinglayer 304B and a bottom dielectric layer 304A. The black color stackalso includes a non-conductive light-absorbing stack 310 havingdielectric layers 304A interleaved with light absorbing layers 304B.

The pigment stack 306 may reverse the positions of dielectric layers204A and 204C of the pigment stack 206, as shown in FIG. 2, such thatthe top layer is dielectric layer 304C, which is different from thedielectric layer 304A used in the non-conductive light-absorbing stack310. Stack 300 further includes a capacitive sensor 208 attached to thebottom of the black color stack 304.

FIG. 4A illustrates a cross-sectional schematic view of opaque region104 in accordance with a third embodiment of the present disclosure.Stack 400 includes the cover substrate 202 on the top and a black colorstack 404 underneath the cover substrate. The black color stack 404includes a pigment stack 406 having a first dielectric layer 404A, alight absorbing layer 404B, and a second dielectric layer 404C. Theblack color stack 404 also includes a non-conductive light-absorbingstack 410 having dielectric layers 404D interleaved with light absorbinglayers 404B. Note that the black color stack 406 may use a differentdielectric layer 406D in the light-absorbing stack 410 from the pigmentstack. This embodiment varies from stack 200. In contrast, stack 200uses dielectric layer 204A in the light-absorbing stack 210, as shown inFIG. 2, where the dielectric 204A in the light-absorbing stack 210 isthe same as one dielectric layer in the pigment stack 206. Stack 400further includes a capacitive sensor 208 attached to the bottom of theblack color stack 304. As previously mentioned, the pigment stack 406may be colored black in order to match the black color of the stack 400,which is under the button 108, to the black ink color of the opaqueregion 104.

The light absorbing layers may include tin film having discontinuousgrain structures, although in other embodiments a different metal ormaterial may be used. Specifically, tin has high light absorption ofvisible wavelengths of light. By using thin and discontinuous grainstructures in layers 204B and/or 304B, a black coating with high opacityand high resistivity may be achieved. The same is true with respect toemploying tin in layer 404B, which may be interleaved with thicknon-conductive or dielectric layers (e.g. SiO₂ layers).

Tin becomes non-conductive when the thickness of a tin layer is keptunder about 100 nm. When the thickness of tin is below this threshold,the tin layer may include or be a discontinuous grain structure, therebypreventing the flow of electrical current. Tin has an electricalresistivity greater than 10⁶ Ωcm when the thickness of tin is less than100 nm (for example, where a 40 nm thick tin layer is used).

By including intervening dielectric layers, the resistivity of the blackcolor stack is significantly increased. For example, the electricalresistivity of tin is much lower than that of Si₃N₄ or SiO₂, and so theintervening dielectric layers may serve to increase the overallresistivity of the color stack. For example, SiO₂ generally has anelectrical resistivity of 10¹⁶ Ωcm, which is slightly higher than theelectrical resistivity of 10¹⁴ Ωcm for Si₃N₄, both of which exceed theresistivity of a thin tin layer. The intervening dielectric layers makesstack 410 non-conductive.

By increasing the number of layers in the black color stack, the opticaldensity of the black color stack can be increased. As an example, onemay use fewer layers to create a gray ink coating, such that the blackcolor stack has a higher optical density than the gray ink coating.

In alternative embodiments, other materials may replace tin in the lightabsorption layers. For example, copper oxide (CuO) generally has goodlight absorption qualities and may form a non-conductive layer, or beused as part of a non-conductive layer. Zinc oxide (ZnO) may also beused as a light absorption layer and likewise has good resistivity.

The dielectric layers in the non-conductive light-absorbing stack and/orthe pigment stack may include silicon nitride (Si₃N₄), silicon oxide(SiO₂), and niobium oxide (Nb₂O₅), among others. These materials haverelatively low dielectric constants compared to black ink. Each of thelayers in the black color stack may vary in thickness or multiple layersmay have uniform thicknesses.

In one example, the top pigment stack may include a layer of siliconnitride over a layer of tin which overlays a layer of silicon oxide. Theblack color stack may include three layers in the pigment stack, and 18tin layers interleaved with 19 silicon nitride layers in thelight-absorbing stack. The black color stack may have a total thicknessof 1.3 μm, generally does not affect the performance of standardcapacitive sensors.

FIG. 4B shows a sample embodiment of a black color stack 200. As shown,the pigment stacks 206, 306, or 406 may include three layers, such as afirst or top Si₃N₄ layer (which may be deposited directly on the coverglass 202), a second Sn layer, and a third SiO₂ layer. Thelight-absorbing stack 210/310/410 may include 19 layers of Si₃N₄interleaved with 18 layers of tin, as shown in FIG. 4B. A bottom layer(which may be a 40^(th) layer) may include Si₃N₄. In some embodiments,the bottom layer may be adjacent to the capacitive sensor 208.

In some embodiments, each or some dielectric layer(s) (e.g., Si₃N₄) mayhave a different thickness than another (or all other) Si₃N₄ layer(s).For example, the 26th, 30th, 32nd, . . . , and 40th layers of Si₃N₄ mayhave a thickness of about 100 nm, while sixth, eighth, tenth . . . , and24th layers of Si₃N₄ many have a thickness of about 30 nm. Continuingthe example, the fourth layer of Si₃N₄ may have a thickness of 10 nm,while the first layer of Si₃N₄ may have a thickness of 40 nm, both ofwhich may be different from the fourth layer or the other Si₃N₄ layersin the light-absorbing stack. In this embodiment, the tin layers eachmay have a thickness of about 5 nm. By varying the thicknesses of thedielectric layers, different wavelengths of light may be absorbed byeach layer. In some embodiments, each of the tin layers may havesubstantially the same thickness.

Various embodiments of the pigment stack 406 are illustrated in FIGS.5A-F. FIG. 5A a pigment stack 506A having Si₃N₄ and SiO₂ layersseparated by a tin layer. FIG. 5B shows a pigment stack 506B havingSi₃N₄ and Nb₂O₅ layers separated by a tin layer. FIG. 5C shows a pigmentstack 506C having SiO₂ and Nb₂O₅ layers separated by a tin layer.

It should be appreciated that the two dielectric layers of FIGS. 5A-Cmay switch positions. For example, FIG. 5D shows that pigment stack 506Dmay include SiO₂ on its top surface in an alternative to the embodimentof FIG. 5A. Similarly, FIG. 5E shows that pigment stack 506E may reversethe position of Si₃N₄ and Nb₂O₅, with Nb₂O₅ on the top surface, in analternative to the embodiment of FIG. 5B. Further, FIG. 5F shows thatpigment stack 506F may reverse the position of SiO₂ and Nb₂O₅ with Nb₂O₅on the top surface, as compared to FIG. 5C. In some embodiments, Sn maybe replaced by other light absorbing materials, such as CuO or ZnO.Also, SiO₂, Nb₂O₅ or Si₃N₄ may be replaced by other dielectricmaterials.

Each of SiO₂, Si₃N₄, and Nb₂O₅ has a lower or significantly lowerdielectric constant than the dielectric constant of certain black inks.For example, SiO₂ has a relatively low dielectric constant, about 3.9,compared to the dielectric constant of black ink. SiO₂ also has goodlight-diffusing properties with a refractive index of about 1.5.

Similarly, niobium oxide (Nb₂O₅) has a relatively low dielectricconstant of about 42. Nb₂O₅ also has a refractive index of about 2.3.Nb₂O₅ is the most common and robust compound of niobium. Additionally,Si₃N₄ has a relatively low dielectric constant of about 7.5, and arefractive index of about 2.

Many embodiments may employ an inorganic dielectric, such as an oxide ornitride, with a relatively low dielectric constant. The dielectriclayers may be deposited by vacuum technology to form very thin films. Insome embodiments, a resin layer may be used.

In some embodiments, the thin opaque non-conductive black coating may beused for the entire opaque region 104 including button 108. Further, thecolor of the button may be matched to the color of the opaque region 104(see FIG. 1). The opaque region 104, in contrast to the button 108 orother area overlying the sensor, may use conventional printing methodsto form a relatively thicker black ink coating including carbon black asa pigment.

By contrast, the button 108 or other area overlying the sensor may usethe thin, opaque, non-conductive black color stack 204, 304 or 404having low dielectric constant, which may have less impact on theperfomance of the capacitive sensor. The pigment stack may also be usedto impart a different color to button 108 as compared to the opaqueregion 104. The color coating under the button 108 may be matched to thecolor of the other opaque region 104, which may include black ink and ismore conductive than the coating under button 108.

A cover glass 202 primarily formed from sapphire may be synthetic ornatural, and in some embodiments may include various forms of alumina.Sapphire is a “hard” substrate material and is generally scratchresistant, and may be more scratch resistant than chemically treatedglass

In various embodiments, the button and/or the cover substrate may beflat, curved, circular, square, and/or rectangular. It will beappreciated by those skilled in the art that the button and/or coversubstrate may vary in shape and/or dimension.

The operation of the capacitive sensor will now be briefly discussed.The capacitive sensor detects a change in capacitance when a user'sappendage (or a suitable object, such as a stylus) approaches or touchesthe sensor. There is a fringe electric field that extends from thecapacitive sensor 208 beyond the cover substrate 202. The electricalenvironment changes when the appendage enters the fringe field, with aportion of the electric field being shunted to ground instead ofterminating at the capacitive sensor. As a result, the capacitance ofthe capacitive sensor 208 decreases, which can be detected.

FIG. 6 illustrates a diagram of a sample a capacitive sensor for sensingfingerprints and/or touch (or near-touch) in accordance with embodimentsof the present disclosure. It should be appreciated that the capacitivesensor is meant as an example only; other sensors (whether capacitive ornot) may be used in various embodiments. For example, pyroelectricsensors, ultrasonic sensors, optical sensors, swipe sensors, and thelike may all be used with embodiments described herein. Accordingly, thesensor of FIG. 6 is but a single, non-limiting example and intended toprovide context for certain embodiments described herein.

The capacitive sensor 208 may be used to provide secure access tosensitive electronic devices and/or data. As shown in FIG. 6, thecapacitive sensor 208 may include both an array of capacitive sensingelements 602 and drive ring 604. The capacitive sensing element 602 mayinclude data or other information with respect to a relatively smallregion of a fingerprint image. Generally, the capacitive sensor 208 maybe used to determine an image of a fingerprint through measuringcapacitance through each capacitive sensing element 602 of thecapacitive sensor 208.

The voltage of the array of capacitive sensing elements 602 is notdirectly driven or modulated, but instead drive ring 604 is modulated bya drive amplifier 606. This modulation, in tum, excites finger 608 andthe voltage and/or charge at each capacitive sensing element 602 variesas drive ring 604 is modulated since finger's 608 voltage changes withthe modulation of drive ring 604.

For the capacitive sensor, the voltage applied to the drive ring 604 maybe limited. Generally, the voltage is no more than a threshold of 4volts (peak-to-peak). Any voltages above this threshold for exciting thefinger 608 may be detected by a person as a “tingling” or uncomfortablefeeling in his or her finger. Although the exact voltage at which onecan sense the tingling may vary from person to person, the 4 voltpeak-to-peak voltage is generally considered as the threshold beyondwhich the uncomfortable feeling is noticeable.

Since the voltage of the drive ring may be restricted to avoid userperception, the thickness of any dielectric overlaying the sensor islimited. Generally, the capacitance between the sensor 208 and finger608 decreases with increased spacing between the sensor and finger orthe thickness of the dielectric layer or stack between the sensor andfinger. For example, when the finger is away from the sensor 208, alower capacitance may be generated between the sensor and finger, andthus lower voltage signals are produced on underlying capacitive sensingelements 602. By contrast, when the finger is closer to the sensor 208,a higher capacitance may be generated between the sensor and finger, andthus higher voltage signals are produced on underlying capacitivesensing elements. With reduced capacitance, the fingerprint image maybecome blurry. As discussed above, by using the thin opaquenon-conductive black color stack with low dielectric constant, theperformance of the sensor may be improved insofar as the materialbetween the sensor and finger (or other sensed object) may provide lesselectrical insulation therebetween.

In embodiments of the present disclosure, the black color stack may befabricated by a deposition method, such as physical vapor deposition,chemical vapor deposition, ion beam deposition, or sputter deposition,among others.

FIG. 7 is a flow chart illustrating steps for fabricating a displaycover, including a black color stack and a sensor positioned adjacentthe black color stack, in accordance with embodiments of the presentdisclosure. Method 700 starts with depositing a cosmetic coating stackon at least one portion of a transparent cover substrate 202, such asbutton 108 (which may be outside the action region of the display 102),at block 702. The transparent cover substrate 202 may be formed of glassor sapphire.

Method 700 continues with depositing a light-absorbing stack over thepigment stack at block 706. The deposition method may include, but isnot limited to, physical vapor deposition (PVD), chemical vapordeposition (CVD), plasma enhanced chemical vapor deposition (PECVD),and/or ion beam assisted deposition (IBAD), among others. Method 700further includes attaching a capacitive sensor 208 to an opposite sideof the black color stack to the transparent cover substrate at block710.

Various techniques are discussed below for forming a black ink coatingon or in opaque regions 104. In these opaque regions, the black inkcoating may be less insulating than the black color stack formed bymethod 700.

A black ink coating may be applied to a glass substrate, for example, inopaque region 104. This black ink coating does not include the button108. For the button 108, black color stack is used instead. The blackink coating include black ink, such as carbon black.

In one embodiment, a silk screen is used. The silk screen includes awoven mesh that transfers ink or printable materials onto a substrate. Afill blade or squeegee is moved across the silk screen, forcing the inkinto the openings of the woven mesh to transfer by capillary actionduring a squeegee stroke. The silk screen method may have issue withcoating thickness uniformity due to the mesh. For example, it maygenerate about 1 μm height difference which may affect the performanceof the capacitor sensor 208.

In another embodiment, a slit coating process may be used. Slit coatingis a process that creates an uninterrupted curtain of fluid that fallsonto a substrate. The substrate is transported on a conveyor belt at aconstant speed through the curtain to ensure an even coat on thesubstrate. The curtain is created by using a slit at the base of aholding tank, such as a metal block, allowing the liquid to fall uponthe substrate.

FIG. 8 illustrates a metal block in accordance with embodiments of thepresent disclosure. As shown, a metal block 802 may include a reservoirmaterial 804, such as black ink, in liquid form. The metal block 802also includes a slit 806 at the bottom. The slit 806 allows black ink804 to pass through to form a coating on a moving substrate. This slitcoating method may have fabrication concerns including voids, surfacecondition, and homogeneity etc.

In a further embodiment, a heat transfer method may be used.Specifically, the heat transfer method uses a carrier film to roll blackink onto the carrier film, and applies the black ink to a glass orsapphire substrate by heating, followed by peeling off the carrier filmfrom the glass or sapphire substrate.

FIG. 9 is a flow chart illustrating steps for applying a black ink layerto a substrate or coated substrate in accordance with embodiments of thepresent disclosure. Method 900 may start with rolling a black inksublayer onto a carrier film, which may be a flexible polymer film, suchas polyethylene(terephthalate) (PET) film, at block 902. Method 900 mayalso include attaching the carrier film with the black ink layerincluding adhesives to a glass or sapphire substrate 202 at block 906.For example, the black ink or pigments may be embedded within theadhesives. Method 900 may further include heating the substrate andcarrier film and applying pressure on the carrier film against thesubstrate 202 at block 910, such that the black ink layer adheres to thesubstrate 202. Method 900 may continue with cooling the substrate andcarrier film to form a coated substrate and peeling off the carrier filmfrom the coated substrate at block 914.

FIG. 10 shows a stack of a carrier film 1002, a first black ink/adhesivesublayer 404A, and a glass or sapphire substrate 202 in accordance withembodiments of the present disclosure. An additional black ink layer maybe applied to the coated glass substrate by repeating the methoddisclosed above for the first black ink layer. This method may providesubstantially homogeneous and uniform opaque thin film.

In yet still another embodiment, spin coating is a procedure that isused to deposit uniform thin films to flat substrates. Generally, asmall amount of coating material is applied on the center of thesubstrate. The substrate is then rotated at high speed in order tospread the coating material by centrifugal force. This spin coatingmethod may create thin films with thicknesses below 10 nm. Therefore,more black ink layers may be deposited to form a black coating, which isdifferent from the stack as shown in FIGS. 2-5.

Processes for depositing a black color stack will now be discussed. Forbutton 106, black color stack is used to make the underneath sensor 208invisible. The various layers of the black color stack 204, 304, 404discussed herein may be formed over the substrate 202 in a variety ofmanners. For example, the technologies may include PVD, CVD, PECVD,and/or IBAD, each producing a slightly different structure to the layer.The different structure may affect electrical properties, opticalproperties of the mirror stack. The deposition of coating materialsvaries by process, with the specific conditions, including theatmosphere, the temperature of the substrate and chamber, the pressure,presence, ratio, type and energy of additional energetic ions, thedeposition rate and the condition of the coating materials, allcontributing to the final structure, composition and density that canaffect the various material properties.

FIG. 11 is a simplified system diagram for deposition system includingPVD in according with embodiments of the present disclosure. Adeposition system 1100 may apply a surface treatment to the substrate202. In this particular example, deposition system 1100 includes one ormore reservoirs 1110 for various coating materials 1108 (e.g., SiO₂,Si₃N₄, Sn, and Nb₂O₅). An inert gas 1112 (e.g., argon or nitrogen) maybe supplied by gas source 1116 through purge or pressurization flow pipe1114, in order to reduce oxidation, wetting and contamination withinreservoirs 1110. Depending on design, reservoirs 1110 may be coupled tovacuum chamber 1118 by one or more delivery tubes 1122, as configured todeliver materials 1108 from reservoirs 1110 to supply systems 1120.Supply systems 1120 utilize a suitable combination of tubes, pumps,valves and other components to direct materials 1108 into vaporizing ordeposition units 1126 for deposition onto substrate 202, as shown inFIG. 2. In the particular configuration of FIG. 11, deposition units1126 are provided in the form CVD or PVD components. Alternatively,other processes and components may be utilized, for example, to treatsubstrate 202 by sputtering, electron beam deposition or electron beamevaporation, IBAD, PECVD or a combination of such processes,

In some embodiments, deposition system 1100 also controls pressure,temperature and humidity to operate chamber 1118 as a vacuum chamber orother chemical or physical vapor deposition environment, Depositionsystem 1100 may also maintain a particular temperature for the surfacecoating process, for example, between about 100° C. and about 150° C.,or between about 100° C. and about 170° C. Air may also be providedwithin chamber 1118, either during or after the coating process, inorder to expose substrate 202 to atmosphere in a controlled process,before removal from chamber 1118.

In general, supply systems 1120 and deposition units 1126 are controlledto deposit selected amounts of material (e.g., SiO₂, Sn, and Nb₂O₅) ontosubstrate 202 in particular orders and combinations.

Having described several embodiments, it will be recognized by thoseskilled in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the disclosure. Additionally, a number of well-known processesand elements have not been described in order to avoid unnecessarilyobscuring the embodiments disclosed herein. Accordingly, the abovedescription should not be taken as limiting the scope of the document.For example, although embodiments herein have been discussed in thecontext of an electronic device, the color stack, methods, and otherembodiments described herein may be used with substantially any productor on substantially any surface.

Those skilled in the art will appreciate that the presently disclosedembodiments teach by way of example and not by limitation. Therefore,the matter contained in the above description or shown in theaccompanying drawings should be interpreted as illustrative and not in alimiting sense. The following claims are intended to cover all genericand specific features described herein, as well as all statements of thescope of the present method and system, which, as a matter of language,might be said to fall therebetween.

What is claimed is:
 1. An electronic device, comprising: a capacitivesensor; a transparent cover layer that covers the capacitive sensor; ablack coating layer interposed between the capacitive sensor and thetransparent cover layer; and a light-absorbing stack interposed betweenthe black coating layer and the capacitive sensor.
 2. The electronicdevice defined in claim 1 wherein the transparent cover layer comprisesa material selected from the group consisting of: glass and sapphire. 3.The electronic device defined in claim 1 wherein the black coating layercomprises a plurality of black pigment sublayers.
 4. The electronicdevice defined in claim 3 wherein the black pigment sublayers all havethe same thickness.
 5. The electronic device defined in claim 3 whereinthe black pigment sublayers have different thicknesses.
 6. Theelectronic device defined in claim 1 wherein the light-absorbing stackcomprises a first layer having a first thickness and a second layerhaving a second thickness that is larger than the first thickness. 7.The electronic device defined in claim 6 wherein the first layercomprises tin.
 8. The electronic device defined in claim 6 wherein thesecond layer comprises a dielectric material.
 9. The electronic devicedefined in claim 8 wherein the dielectric material comprises a materialselected from the group consisting of: silicon dioxide, silicon nitride,and niobium oxide.
 10. An electronic device, comprising: a transparentlayer having an inner surface; a black coating on the inner surface; atouch sensor behind the transparent layer, wherein the black coatingcompletely covers the touch sensor; and a light-absorbing stackinterposed between the black coating and the touch sensor, wherein thelight-absorbing stack absorbs light that passes through the blackcoating.
 11. The electronic device defined in claim 10 wherein thelight-absorbing stack comprises a metal layer and a dielectric layer.12. The electronic device defined in claim 11 wherein the metal layercomprises tin.
 13. The electronic device defined in claim 11 wherein thedielectric layer comprises a material selected from the group consistingof: silicon dioxide, silicon nitride, and niobium oxide.
 14. Theelectronic device defined in claim 11 wherein the dielectric layer isthicker than the metal layer.
 15. The electronic device defined in claim10 wherein the black coating comprises dielectric layers interleavedwith metal layers.
 16. An electronic device, comprising: a fingerprintsensor; a transparent layer that covers the fingerprint sensor; a blackink layer that covers the fingerprint sensor; and a light-absorbingstack interposed between the fingerprint sensor and the transparentlayer.
 17. The electronic device defined in claim 16 wherein thelight-absorbing stack comprises metal layers interleaved with dielectriclayers.
 18. The electronic device defined in claim 17 wherein the metallayers comprise tin.
 19. The electronic device defined in claim 18wherein the dielectric layers comprise a material selected from thegroup consisting of: silicon dioxide, silicon nitride, and niobiumoxide.
 20. The electronic device defined in claim 19 wherein thedielectric layers are thicker than the metal layers.